1. Field of the Invention
The present invention relates to a method of and an apparatus for optically detecting presence or absence of a photosensitive object, such as a photographic process film, at a prescribed position of a path carrying the photosensitive object.
2. Description of the Prior Art
In the field of photographic process or the like, it is generally required to automatically convey a photographic film to a setting position. Therefore, employed is a photosensitive object detecting apparatus for detecting presence or absence of the film at a prescribed position thereby to detect the position and width of the film. Such a photosensitive detecting apparatus is constructed in a contact type apparatus employing a microswitch etc. or in a non-contact type apparatus employing light or supersonic waves. Within these types of apparatus, the former tends to spoil the film.
Therefore, generally employed is a photosensitive object detecting apparatus of the non-contact type, which is most importantly represented by an apparatus of an optical type capable of accurate detection. In the apparatus of the optical type, however, detection light must necessarily be applied to the photosensitive object, whereby a latent image is inevitably sensitized by the detection light. In order to prevent this sensitization, detection light outside a photosensitive wavelength band of the film may be employed, and hence near infrared rays are mainly employed as detection light for a film for general use. However, image process equipment utilizing semiconductor laser etc. employ an infrared-photosensitive film, such as a process infrared film, which is inevitably sensitized if ordinary near infrared light is utilized.
However, a light emitting element which emits light having a non-photosensitive wavelength with respect to such a film (wavelength of a far-infrared band for a process infrared film, for example) is generally high-priced, so that the cost of photosensitive object detecting apparatus employing the same is increased.